Self-penetrating fastening system

ABSTRACT

The self-penetrating fastening device proposed consists of a rivet ( 3 ), acted on by a punch ( 13 ), and a die ( 15 ), the rivet consisting of a head ( 4 ) and a shank ( 6 ) with a central cavity ( 8 ) whose open end partly penetrates a metal sheet ( 1 ) in which it is held. At the end of the rivet shank ( 6 ) remote from the rivet head ( 4 ), the cavity ( 8 ) is essentially conical in shape, the angle of the aperture being α. In addition, the transition zone between the rivet shaft ( 6 ) and the underneath ( 11 ) of the rivet head ( 4 ) is essentially rounded (R 1 ). A rivet joint produced using this device is sufficiently strong to meet severe requirements, and the surface of the metal sheet in the region of the rivet is not affected by the perforation process.

The invention relates to a self-penetrating fastening system including arivet, a punch and a pressure pad (die), the rivet consisting of a headand shank with central cavity whose free end surface does not completelypenetrate a workpiece (metal plate) receiving it.

A self-penetrating fastening system such as this is described in GermanPatent DE 39 42 482 C1. In the embodiment explained, two plates ofdifferent thicknesses, parts of the wall of a container, are connectedto each other, particular value being attached to water and gastightness. Corrosion stability is also taken into account in this stateof the art.

If such self-penetrating fastening systems are also to be used in thearea of automotive technology, for example in order to connect bodysheets together, other essential factors are involved in addition to thecorrosion stability in question. Because of the high mechanical stressesoccurring, the strength of the connection of two plates is of decisiveimportance. In addition, depending on the place where the connection isapplied (uncovered body areas), it should be possible to make the top ofthe rivet head as nearly flush as possible with the surrounding platesurface after manufacture. The process of producing the rivetedconnection should harm the surface areas in question as little aspossible (e.g., through deformation resulting from warping of thesheet), so as to hold minimum any finishing operations required toimprove the appearance.

The state-of-the-art riveted connection does not meet these additionalrequirements to the extent desired.

Hence it is the object of the invention to develop the state-of-the-artself-penetrating fastening device in such a way that the rivetedconnection produced by the device can meet high requirements withrespect to its strength and so that the surface of the workpiece willnot be permanently impaired.

It is claimed for the invention that this is accomplished in that theself-penetrating fastening system possesses the additional featurespresented in the descriptive portion of claim 1.

As a result of the design of the shank cavity in the area of the freeend surface in the area of the free end surface, the cavity beingconical in the broadest sense of the word, as the rivet penetrates aworkpiece more deeply the resulting higher expansion force causes theradial movement of the rivet shank outward (effects expansion) to reachan order of magnitude such that an expansion angle >45° may be reached.On the other hand, this process results in an undercut increasing thestrength, the direct tensile strength in particular (end area of therivet shank projecting radially further than the convexity of the lowersheet adjoining the jacket of the free end surface and the more or lessrounded transition of the shaft into the rivet head during riveting alsoresult in the gentlest possible treatment of the upper sheet duringpenetration of an upper sheet as illustrated in the embodiment, and alsomake certain that, among other things, if the rivet head has a flat top,this top can be embedded so as to be flush with the surrounding surfaceof the sheet and so that the circular notch marking the transition ofrivet head to sheet surface is very narrow and extremely shallow.

Supplementary developments of the invention are described in thedependent claims. In addition to the preferred embodiment indicated, itis to be regarded as a special development (claim 6) that thecircumferential wall of the die cavity is designed so as to be radiallymovable. Especially in the case of thin sheets this involves asupporting effect during cutting of the top sheet, without the processof spreading of the rivet shaft in the bottom plate being impaired inany way whatever, since the spring-mounted groove blocks (claim 7) canmove radially outward as required.

Preferred embodiments of the object of the invention are shown in thedrawing and are explained in what follows with reference to appropriateparts of the drawing, in which

FIG. 1 shows a rivet as a component of the self-penetrating fasteningdevice,

FIG. 2 the layout of the individual elements (section of a multipart diealong arrows II in FIG. 3) of the self-penetrating fastening device,along with the metal sheets to be connected which are received betweenthese elements,

FIG. 3 a top view of the multipart die, and

FIG. 4 a riveted joint made on the basis of the invention.

As is revealed by examination of the figures as a whole, a top and abottom metal sheet 1,2 are joined together by means of a rivet 3. Thelatter consists of a rivet head with flat top 5 and a cylindrical shank6 with central (longitudinal axis 7) cavity 8.

In the area of the shank 6 facing away from the rivet heat 4 the cavity8 is in the form of a cone 9 with aperture angle α, the sides 10 of thecone preferably being slightly convex. A “sharp edge” is virtuallycreated as a result within the limits of the possibilities created byproduction technology; during stamping the rivet can penetrate the topmetal sheet 1 by way of this edge and be embedded in the bottom metalsheet 2.

In addition to the cone aperture angle a, the shaping and dimensioningparameters of the rivet 3 as shown in FIG. 1 are the diameter d1 of thecavity 8, the diameter d2 of the rivet shank 6, the diameter D of therivet head 4, the total length L of the rivet 3, the height H of therivet head 4, the outwardly directed radius R1 for the transitionbetween rivet shank 6 and bottom 11 of rivet head 4, the inwardlydirected and adjoining radius R2 for the transition between rivet headbottom 11 and rivet head top 5, and radius R3 for the bottom of cavity8. The height H of the rivet head 4 is determined from the beginning oftransition from rivet shank 6 to outwardly directed radius R1.

The depth of the cavity 8 is such that its rounded bottom 12 (radius R3)extends into the rivet head 4 within the limits of the possibilitiesprovided by the production technology.

Radius R1 prevents the occurrence of high peak stresses in the rivethead area during the perforation process, inasmuch as there are no sharpedges present.

The cone aperture angle α is to be selected preferably as a function ofthe properties of the material (yield point R_(p0,2)) of the metalsheets 1,2 to be joined. The following correspondences have been foundto be advantageous:

R_(p0,2)≧250 N/mm² ≅75°<α<120°

R_(p0,2)<250 N/mm² ≅25°<α<75°

The other components of the self-penetrating fastening device are showndiagrammatically in FIG. 2. The rivet 3 is impacted by a punching forceF_(s) applied by a punch 13 positioned on the top 5 of the rivet head 4.The punch 13 is surrounded by a pressure pad 14, which is in contactwith the metal sheets 1,2 together with a die 15 functioning ascounterpressure tool and has a pressure pad force F_(N) applied to it.This pressure pad force F_(N) should be kept constant during the rivetperforation process; this can be suitably accomplished by causing thepressure pad 14 to have force applied to it by a suitablepressure-controlled hydraulic unit.

The top surface of the pressure pad 14 in contact with the top metalsheet is roughened in order to optimize the action of this pad. As aresult, excessive drawing of the surrounding sheet areas into theperforation zone during perforation can be prevented; instead, the topmetal sheet 1 is cut cleanly.

The multipart die 15 illustrated in FIGS. 2 and 3 has a drift 20 withpunching stud 16. The drift 20 is received into a suitably dimensionedcavity 17 of a drift recess 18 and held there by a setscrew 19introduced into the cavity. The punching stud 16 can be caused toproject a greater or lesser distance beyond the top 22 of the driftrecess 18 by a shim 21 of specific thickness, one which can easily bereplaced by another shim of a different thickness. A vent 31 made in thedrift recess 18 extends to the cavity 17.

Three radially oriented grooves 23 evenly distributed over thecircumference are made in the top 22 of the drift recess 18, each groovereceiving a block 24. The ends of the groove blocks 24 facing punchingstud 16 are curved in shape and complement each other to form a closedcircle.

The radially outer end of each groove block 24 has force applied to itby a leaf spring 25 which is fastened to the drift recess 18 by pan headscrews 26. Lastly, a pin 27 projecting downward from the drift recess 18serves to receive and immobilize it in a tool.

During the riveting process the groove blocks 24 are forced outward bythe material displaced by the rivet 3, that of the bottom metal sheet 2in particular, so that an additional cavity promoting the spreadingprocess is ultimately provided for the rivet 3. After the joint has beenproduced and the resulting bond material removed from the die, the leafsprings 25 force the groove blocks back into their initial position.

It is essential for the diameter of the die (circle 28 formed by thegroove blocks 24) to be at the beginning of the joining process onlyslightly larger than the outer diameter d₂ of the rivet shank 6, so thatthe perforation of the top of the metal sheet 1 facing the rivet will beas clean as possible without the metal sheet being drawn into the die15.

Lastly, FIG. 4 shows a finished riveted joint. It is made especiallyclear here that the top 5 of the rivet head 4 is flush with the top ofthe top metal sheet 1. It is also to be seen that the circularindentation 29 in the transitional area between rivet head 4 and topmetal sheet 1 is quite small and shallow. FIG. 4 also shows the largeundercut 30 greatly enhancing the strength of the riveted joint. Thisundercut 30 results among other things from the large rivet shankexpansion angle β (>45° if necessary) formed with the rivet 3 duringperforation.

The self-penetrating fastening device may, of course, also be used tojoin more than two sheets, or even to fasten only one rivet, forinstance one with a bolt head, in a single sheet.

Nor is application restricted to the joining of metal plates or metalsections. On the contrary, nonmetallic workpieces (e.g., ones ofplastic) may also be joined by means of this device.

What is claimed is:
 1. A self-penetrating fastening system including arivet, a punch and a counterpressure tool, wherein: said rivet includesa rivet head and a rivet shank, said rivet shank including a centercavity and a free end remote from said rivet bead that does not fullypenetrate a workpiece receiving said shank during a rivet operation;said rivet shank free end remote from said head including a cylindricaloutside wall surface and a substantially conical inside wall surfacehaving an aperture angle; and including a substantially roundedtransition region having a radius and extending from said shank to anupper portion of said rivet head.
 2. A system as in claim 1 wherein saidrivet head includes a flat top for joining flush with a top of aworkpiece after completion of said rivet operation.
 3. A system as inclaim 1 wherein said rivet shank expands to an expansion angle β>45°during said rivet operation.
 4. A system as in claim 1 wherein saidcounterpressure tool includes a roughened surface for contact with aworkpiece in an area surrounding said punch.
 5. A system as in claim 1wherein said counterpressure tool includes a cavity for receiving anexpanding rivet shank, said tool cavity including a radially moveablecircumferential wall, and said counterpressure tool includes a driftsupport, grooves on a top side of said drift support facing a workpiece,groove blocks guided in said grooves, and springs for pressuring saidgroove blocks.
 6. A system as in claim 5 wherein said counterpressuretool includes a drift support, a central cavity in said drift supportfor holding a drift, and three radially oriented grove blocks evenlydistributed over the circumference of said drift support and includingends that face said drift and complement each other to form a circle. 7.A system as in claim 1 wherein said rivet head has a height determinedfrom a beginning of said transition region to said upper portion of saidrivet head over a second transition region having a radius, and whereinsaid center cavity includes a bottom that extends into said height ofsaid rivet head.
 8. A system as in claim 1 wherein said counterpressuretool includes a cavity for receiving an expanding rivet shank, said toolcavity including a radially movable circumferential wall.